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Want to know my lenses real speed.
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PostPosted: Thu Jun 16, 2022 5:00 pm    Post subject: Want to know my lenses real speed. Reply with quote
Hi guys. I've noticed some of my prime lenses given for the same speed don't give the same exposure at all WO (with same shutter speed and ISO) . I compared zuiko 50 f1.2 to rokkor 58 1.2. The zuiko is about a full stop brighter than the MD. My Zeiss C/Y 50 1.4 is almost half a stop faster than Takumar 1,4 Can someone please explain ? Thank you!


PostPosted: Thu Jun 16, 2022 6:44 pm    Post subject: Re: Want to know my lenses real speed. Reply with quote
Phalbert wrote:
Hi guys. I've noticed some of my prime lenses given for the same speed don't give the same exposure at all WO (with same shutter speed and ISO) . I compared zuiko 50 f1.2 to rokkor 58 1.2. The zuiko is about a full stop brighter than the MD. My Zeiss C/Y 50 1.4 is almost half a stop faster than Takumar 1,4 Can someone please explain ? Thank you!


There is a T-stop value for this. Lenses transmit different amounts of light depending on design and coating. There is also the issue of lens makers being liberal with the f-stop and focal length claims. Radioactive lenses with yellow glass also transmit less light than they should.

The T-stop is always higher than the F-stop.


PostPosted: Thu Jun 16, 2022 7:12 pm    Post subject: Reply with quote
Then, is it so that the T stop is the real measurable amount of light going through a lens, and the f stop is the calculation made for each lens with focal length through actual max opening size? I assumed each max f stop was equal to all the others for a given focal length. Not so? So different 50mm f 1,4 lenses can / will let significant different amounts of light go through? If that is so, maybe some 1,4 are in fact faster than some f1, 2 ?


PostPosted: Thu Jun 16, 2022 8:26 pm    Post subject: Reply with quote
Well, it depends on your interpretation of "faster".

The f-stop is simply the ratio of the focal length of the lens to the diameter of the entrance pupil (or the reciprocal, depending which way you quote it), and therefore a purely geometric measure. If you are talking about the (relative) brightness of a lens, then indeed you want the T-stop as blofaton already mentioned, which accounts for transmission losses due to the types of glass used, coatings, etc.

This is why in the old film days where an external light meter was used, the photographers would soon become familiar with their lenses and know which of them needed some adjustment to the exposure indicated by the light meter because their T-stop wasn't quite the same as their f-stop.

If you have an early variant of the Minolta MC ROKKOR 58mm/1.2 then check whether there is a noticeable yellowing when looking through the lens. If there is noticeable yellowing then almost certainly you have an early copy with radioactive thorium glass, which will gradually have reduced the transmission somewhat over the decades...


PostPosted: Thu Jun 16, 2022 8:56 pm    Post subject: Reply with quote
Light Transmission meter required:

https://www.qualtechproductsindustry.com/products/appearance-testing-instruments/reflection-opacity-testing-instruments/light-transmission-meter/
https://www.worldoftest.com/light-transmittance-and-haze-tester-qt-hz

Apparently DXOMark lists T-stops in lens database. https://www.dxomark.com/glossary/transmission-light-transmission/


PostPosted: Thu Jun 16, 2022 10:01 pm    Post subject: Reply with quote
Phalbert wrote:
Then, is it so that the T stop is the real measurable amount of light going through a lens, and the f stop is the calculation made for each lens with focal length through actual max opening size? I assumed each max f stop was equal to all the others for a given focal length. Not so? So different 50mm f 1,4 lenses can / will let significant different amounts of light go through? If that is so, maybe some 1,4 are in fact faster than some f1, 2 ?


I don't know enough to answer the technical stuff.

But you can get some examples from manufacturers that make photography lenses and cine versions with the same optics.
It is more important with T-values in film making.

If we look at Samyang here are some examples:

135mm f2 T2.2
50mm f1.4 T1.5
14mm f2.8 T3.1

I can also recommend looking at patents to see that the number printed on the lens is not always exactly the true values.

From Sonyalpharumors:
https://www.sonyalpharumors.com/new-sony-patents-discloses-details-about-the-new-135mm-f-1-8-and-100mm-f-1-4-fe-lenses/

The 135mm f1.8 is actually 131mm f1.85. But I'm not sure that is the final lens they ended up making but this is how it looks for most lenses.


PostPosted: Fri Jun 17, 2022 5:46 am    Post subject: Reply with quote
How did you measure the exposure differences with your lenses ? By comparing exposure values in the viewfinder while viewing the same subject in constant light ? Or by comparing luminosity in the center of the images ? Apart from different light transmission values you might as well take into account the amount of vignetting of different lenses since stronger vignetting causes the lightmeter to choose a longer exposure time.


PostPosted: Fri Jun 17, 2022 8:34 am    Post subject: Reply with quote
Alsatian2017 wrote:
How did you measure the exposure differences with your lenses ? By comparing exposure values in the viewfinder while viewing the same subject in constant light ? Or by comparing luminosity in the center of the images ? Apart from different light transmission values you might as well take into account the amount of vignetting of different lenses since stronger vignetting causes the lightmeter to choose a longer exposure time.


Like 1 small I would second this.

If you are using a camera with focus aids embedded in the focus screen (split prism / microprisms / acute matte etc.) then the position of the exit pupil of the lens may also affect the reading; with standard (50mm~ish on FF) lenses that is not usually an issue though. For cameras with interchangeable focus screens the manufacturers would usually provide a table with the required exposure compensation for this, sometimes even for specific focus screen/lens combinations.


PostPosted: Fri Jun 17, 2022 8:50 am    Post subject: Reply with quote
visualopsins wrote:
Light Transmission meter required:

https://www.qualtechproductsindustry.com/products/appearance-testing-instruments/reflection-opacity-testing-instruments/light-transmission-meter/
https://www.worldoftest.com/light-transmittance-and-haze-tester-qt-hz

Apparently DXOMark lists T-stops in lens database. https://www.dxomark.com/glossary/transmission-light-transmission/


If you are less interested in the spectral transmission characterisitcs you can also rig up something yourself with a cheaper external light meter, especially one which has an attachment for taking spot-readings off a ground glass screen. You can then take a central spot-reading through the lens (given an appropriate external light source of constant and even illumination), and compare it with the reading taken through a known glass-less aperture you create yourself equal to the lens' stated f-stop. Not as precise but it should give you some idea of what the lens' T-stop is.


PostPosted: Fri Jun 17, 2022 8:56 am    Post subject: Reply with quote
Thanks to all of you. I think I have it figured out. Rokkordoctor, by faster I mean more light and fastest possible shutter speed, especially in dim situations. Yes my 58 is indeed a MC rokkor PG and there is indeed a visible yellowing, but it doesn't look bad to me. (I never noticed that, while it was very obvious on the takumar 7 els) Maybe less than 1/3 of a stop. Ok. Thanks again.


PostPosted: Fri Jun 17, 2022 9:09 am    Post subject: Reply with quote
Alsatian, (I'm one too 😊😉) I just missed your post. I didn't make any accurate measurement. I took pictures with the same light, speed and ISO in manual mode, and compared the brightness, but I know more or less by experience what one stop difference looks like. This is all made with 1,6 x dslr sensors, so no visible vignetting.


PostPosted: Fri Jun 17, 2022 9:18 am    Post subject: Reply with quote
Phalbert wrote:
Thanks to all of you. I think I have it figured out. Rokkordoctor, by faster I mean more light and fastest possible shutter speed, especially in dim situations. Yes my 58 is indeed a MC rokkor PG and there is indeed a visible yellowing, but it doesn't look bad to me. (I never noticed that, while it was very obvious on the takumar 7 els) Maybe less than 1/3 of a stop. Ok. Thanks again.


Long exposure to UV light can largely clear this yellowing, which works better when the thorium element is taken out and exposed directly to UV light. I would advise against taking out the radioactive element unless you know how to service lenses. The element is safe to handle with limited contact when intact, but any damage to the glass would pose a potentially serious radiation health risk if small shards enter the (human) body.

The whole (assembled) lens can be exposed to UV light to clear the yellowing, but it will take much longer.

Be very careful when exposing this lens to direct sunlight to clear the yellowing; it is a potent magnifier (of f/1.2!!) and can cause a fire!! If you do, please make sure no (flammable) object is in or near the focal plane for any extended period.


PostPosted: Fri Jun 17, 2022 10:06 am    Post subject: Reply with quote
visualopsins wrote:
Light Transmission meter required:

https://www.qualtechproductsindustry.com/products/appearance-testing-instruments/reflection-opacity-testing-instruments/light-transmission-meter/
https://www.worldoftest.com/light-transmittance-and-haze-tester-qt-hz

Apparently DXOMark lists T-stops in lens database. https://www.dxomark.com/glossary/transmission-light-transmission/


I've tested a number of my lenses using a research spectrometer (Able to measure each wavelength in turn from 190nm to 1100nm). I've found it helpful for the variation with wavelength (especially in the UV), but it doesn't really tell anything about the total light gathered by a lens. The test beam is only a few millimeters square, so alignment is critical, especially for short focal lengths which bend the beam more.
A well aligned multicoated telephoto lens can give me up to about 98% transmission even when the maximum aperture is quite small.
With some other lenses I've been unable to get a maximum transmission over 20%

Doing it properly would require a lens mount to keep the lens centred & an integrating sphere to include all light bent off to the side so that it misses the (relatively small) detector. I definitely cant justify the integrating sphere & the lens mount involves more DIY than I can be bothered with.

Where total light is desired surely comparison of TTL exposure with an external lightmeter would do the job. Metering off a large uniform & evenly lit surface is probably best, but a properly warmed up LCD screen would do.
This would cost less than an integrating sphere (let alone the spectrometer) or one of the light meters you linked to. It also avoids the issue of significant stray light (mounting the lens on the detector) & allows a lens of any size that can be mounted on the camera....


PostPosted: Fri Jun 17, 2022 10:23 am    Post subject: Reply with quote
Rokkordoctor, thanks for the warnings. I already burned a few holes in various items trying to de-yellow a takumar 50/1,4. But it worked in the end. 😊😊😊


PostPosted: Fri Jun 17, 2022 2:50 pm    Post subject: Re: Want to know my lenses real speed. Reply with quote
Phalbert wrote:
Hi guys. I've noticed some of my prime lenses given for the same speed don't give the same exposure at all WO (with same shutter speed and ISO) . I compared zuiko 50 f1.2 to rokkor 58 1.2. The zuiko is about a full stop brighter than the MD. My Zeiss C/Y 50 1.4 is almost half a stop faster than Takumar 1,4 Can someone please explain ? Thank you!



#1


#2


#3


#4


No Takumar in my archive. Notice that the measured F stop can deviate from the one written on the beauty ring and The T stop will always be another number and more related to your exposure tests. With vintage lenses there are other shifts possible due to age and "repairs".


PostPosted: Sat Jun 18, 2022 11:43 am    Post subject: Reply with quote
The aperture is a numerical value based on the focal length and width of the exit pupil. Light transmission of lenses with similar focal lengths and apertures vary because of the number of elements and type of coating. Every surface in a lens loses light. Movie lens apertures are measured for transmission and the apertures there are known as T stops. Each lens at a given t stop will transmit the same light.

In the 1930s when coated lenses were becoming the norm, a famous journalist used a 5cm f1.5 and a 35mm f2 lens at full aperture and the same shutter speed and found that the exposures were identical. The 5cm f1.5 was uncoated and the 35mm was coated.


PostPosted: Sat Jun 18, 2022 1:38 pm    Post subject: Reply with quote
philslizzy wrote:
The aperture is a numerical value based on the focal length and width of the exit pupil.


Not quite. The aperture is the adjustable mechanism itself, usually an iris. The f-stop is the numerical ratio of the focal length to the diameter of the entrance pupil, not the exit pupil.


PostPosted: Sat Jun 18, 2022 3:06 pm    Post subject: Reply with quote
It's a bit odd to have to say this...

A little bit of actual book reading would eliminate the need for this type of discussion.
The topic is covered in quite masterful detail in the old Focal/Ilford press "Manual Of Photography".

-D.S.


PostPosted: Sat Jun 18, 2022 3:43 pm    Post subject: Reply with quote
For reading: https://www.cambridgeincolour.com/tutorials.htm


PostPosted: Sat Jun 18, 2022 6:44 pm    Post subject: Reply with quote
Doc Sharptail wrote:
It's a bit odd to have to say this...

A little bit of actual book reading would eliminate the need for this type of discussion.
The topic is covered in quite masterful detail in the old Focal/Ilford press "Manual Of Photography".

-D.S.


I agree 100%. There are many books that cover this subject in great detail, as well as many other theoretical photography topics that seem to be the subject of so much confusion on these forums.

But dare I suggest we may be looking at a generational difference here in attitude towards reading well written and edited technical books with an index, vs. using a search engine for everything on the wild-west of information that is the internet Wink.

I'm old school myself; I would rather spend $100 on an authoritative written scientific book than spend countless hours trying to make sense of all contradicting info to be found on the internet. Each to their own Wink


PostPosted: Sat Jun 18, 2022 8:18 pm    Post subject: Reply with quote
Perhaps you have a valid point there.

When I first picked up the Focal book, I found the mathematical formulas presented as a bit intimidating, and challenging.
Some of the formulas are still beyond my grasp.
The theoretical points are actually as clear as the type-face they're set in- some after 5 or 6 readings.

-D.S.


PostPosted: Sun Jun 19, 2022 7:07 am    Post subject: Reply with quote
A lot of learning material here. Great infos. Thanks to all.


PostPosted: Mon Jun 20, 2022 1:01 pm    Post subject: Reply with quote
blotafton wrote:
...
I can also recommend looking at patents to see that the number printed on the lens is not always exactly the true values.

From Sonyalpharumors:
https://www.sonyalpharumors.com/new-sony-patents-discloses-details-about-the-new-135mm-f-1-8-and-100mm-f-1-4-fe-lenses/

The 135mm f1.8 is actually 131mm f1.85. But I'm not sure that is the final lens they ended up making but this is how it looks for most lenses.


As I work as optics designer a bit of warning: It is exolicit mentioned in at least in one of the lens design books I have read, ans alos heard this: The lens patents are deliberately NOT the real design of the lens. Perhaps this was different n the good old days of lens design, but today the companies dont give away the exact design they want to use with all refinment.


PostPosted: Mon Jun 20, 2022 1:18 pm    Post subject: Reply with quote
ZoneV wrote:
blotafton wrote:
...
I can also recommend looking at patents to see that the number printed on the lens is not always exactly the true values.

From Sonyalpharumors:
https://www.sonyalpharumors.com/new-sony-patents-discloses-details-about-the-new-135mm-f-1-8-and-100mm-f-1-4-fe-lenses/

The 135mm f1.8 is actually 131mm f1.85. But I'm not sure that is the final lens they ended up making but this is how it looks for most lenses.


As I work as optics designer a bit of warning: It is exolicit mentioned in at least in one of the lens design books I have read, ans alos heard this: The lens patents are deliberately NOT the real design of the lens. Perhaps this was different n the good old days of lens design, but today the companies dont give away the exact design they want to use with all refinment.


Not exactly the topic but there is a user on the dpreview forum that compares the schematics published by the lens makers with his own that I think he makes by inputting data from the patents into a design software to see varying degrees of accuracy. https://www.dpreview.com/forums/thread/4628869

So it seems to be true that the lens makers don't want to show the finer details.


PostPosted: Tue Jun 21, 2022 8:14 am    Post subject: Reply with quote
yes thats true just came out from a test with zeiss 50/1.4 or 85/1.4- the f1.4 is at least 1/3- 1/2 stop darker clearly. in other words zeiss was cheating. will investigate further. one can also adjust light and change aperture. this way shutter-accuracy is not involved. best would be using film and resp. greyscale-card-and densitometer. i can then also check shutter-accuracy.
and digital cam shutter to be verified or tested the same way like with analog cam.
i compared f 1.4 to a f 1.5. f1.5 is about 1/3 darker.
yes i tested center with eye. its darker clearly. same discussion in a german forum.
just wanted to ask about the 135/1.8.
i have both CY 50/1.4 and Super Takumar 50/1.4 so i will compare asap.
.....................................................................................................
i am currently testing and building FC-adapters. first on EOS 60D. later when i have GG by brightscreen -he must make-also with analoge EOS.
C/Y-EOS, Ni-EOS. M42-EOS- that is a different weird story. many problems. if will be finally the lushkov-dandelion to be used since all others have limits. has eTTL2. but seller is offline or not selling since there is war in UA.
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